1. Field of the Invention
The present invention relates to a humidity sensitive device, and more specifically to an improvement in a material composition of a humidity sensitive film.
2. Description of the Prior Art
A humidity sensitive device is typically structured such that opposing electrodes are formed on an insulating substrate and a humidity sensitive film is formed on the surface of the insulating substrate and at least between the opposing electrodes. The humidity sensitive film comprises a material exhibiting a predetermined humidity-resistance value characteristic. Accordingly, the inherent resistance value of the humidity sensitive film is variable as a function of an ambient humidity and as a result a humidity condition can be determined in terms of a resistance value of the humidity sensitive device.
One example of prior art humidity sensitive devices employs an organic high polymer film such as cellulose. However, a disadvantage is encountered in this example that an organic material is liable to be deteriorated during the lapse of time and hence a stabilized characteristic can not be attained.
Another example of prior art humidity sensitive devices employs a metal oxide such as Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, and the like as a humidity sensitive film. A humidity sensitive film is formed by means of evaporation, sputtering, printing and the like of such metal oxide. It has been observed that a thin film of such metal oxide is superior in absorption property. A variation of humidity is detected in terms of a change of an electrical resistance value by virtue of absorption moisture. An advantage is brought about by such a humidity sensitive film that a response is fast. Nevertheless, a disadvantage is encountered in that a resistance value is relatively high and measurement of a low humidity is rather difficult.
A third typical example of conventional humidity sensitive devices utilizes a ceramic semiconductor. Although a ceramic semiconductor has a feature of excellent heat resisting property, a disadvantage is encountered in that a specific resistivity is high and the cost is expensive as compared with that in case of a humidity sensitive film using an organic resistance film of such material as enumerated in the first example.